ruby (n.)

"clear rich-red variety of corundum," c. 1300, from Old French rubi (12c.), from Medieval Latin rubinus lapis "red stone" (source also of Italian rubino), from Latin rubeus "red," related to ruber, from PIE root *reudh- "red, ruddy." As a color name from 1570s. As an adjective from late 15c. Modern French rubis is not explained; Klein suggests a plural mistaken for singular.